Active thermography is used to nondestructively evaluate (NDE) samples in order to detect sub-surface defects. It is effective for uncovering internal bond discontinuities, delaminations, voids, inclusions and other structural defects that are not detectable by visual inspection of the sample. Generally, active thermography involves heating or cooling the sample to create a difference between the temperature of the sample and the ambient temperature and, then observing the infrared thermal signature that emanates from the sample as it returns to a state of thermal equilibrium. Pulsed thermography is widely used in the nondestructive evaluation of component parts used in aerospace and the power generation industry.
An infrared (IR) camera is typically used for thermography to measure the infrared radiation emitted from a sample as it returns toward a steady state temperature. Anomalies in the cooling behavior of the sample are produced when sub-surface defects are present because the sub-surface defects affect the diffusion of heat from the surface of the sample into the body of the sample. In particular, sub-surface defects cause the surface immediately above the defect to cool at a different rate than that of the surrounding (defect-free) areas. As the sample cools, the IR camera captures and records an infrared image of the sample, creating a sequential, time record of the sample's surface temperature.
In performing thermography, it is typically assumed that the integration time, (i.e. the time during which photons are collected by the focal plane array (FPA) detector of the infrared camera), occurs simultaneously with the beginning (i.e. on-set) of the video frame, or, more specifically, with the on-set of frame synchronization pulse (hereinafter frame sync or vertical sync signal). In fact, in many high performance IR cameras typically used in NDT applications, the integration time precedes the frame sync by a percentage of the frame sync period. For example, the integration for a given time frame may occur during the outputting of the previous frame (commonly referred to as “integrate while read mode”). The precise time at which the temperature measurement is made may differ from the apparent time (based on the frame number) by a significant amount. This difference is especially acute in the earliest post-flash frames.
The present invention uses an infrared camera to accurately measure the on-set of the flash event pulse with respect to the frame sync signal. The present invention also uses the infrared camera to measure the duration of the flash pulse event. This is accomplished by detecting a slight disturbance in the pixel values in the frame that is read-out concurrently with the occurrence of the heating pulse.